Svenska:

Den här webbplatsen kan endast nås som arkivreferens. De officiella webbsidorna för avdelningen för partikelfysik finns på http://particlephysics.physics.lu.se

Sidan nedan slutar uppdateras fr.o.m. 2020-09-22 och uppfyller endast delvis tillgänglighetsdirektivet. Se tillgänglighetsredogörelsen

English:

The webpage you reached is made available for archiving purposes only. The official division of particle physics webpages can be found at http://particlephysics.physics.lu.se

The page below is not updated any longer since September 22, 2020 and does not conform completely to the accessibility regulation. See the accessibility statement

Background coincidences

The off-energy electron background disturbs the VSAT data in many ways, as will be discussed in section 4.1 the coincidence of two off-energy electrons can trigger a Bhabha event. This ``false Bhabha'' rate normally stays below 30% of the number of Bhabha triggers, but can at times go above 50% (Fig. 3.6). Most of the false Bhabhas can be removed with narrow cuts due to the well confined properties of a true Bhabha event. The remaining background can be quantized by a linear combination of four different measurements [16]:

E_cut/($${\frac{S_1\cdot S_2}{SE_1 SE_2}}$$ - 1)        $${\frac{SE_1 \cdot SE_2}{\Delta T}}$$        B_soft - B_hard        FB_scal

Here E_cut is the number of events cut away by the Bhabha energy cut, S and SE the number of downscaled off-energy electron triggers with and without the energy cut. The duration of the measured data is given by $\Delta$T and B_soft - B_hard is the difference between the normal Bhabha sample and one with especially hard background cuts. Finally the FB_scal is the online delayed Bhabha scaler (taken as the coincidence of two opposite hits separated by four bunch crossings), which only is usable in cases of bad statistics.

A $\gamma$$\gamma$-event is triggered by cuts on the hadronic system in the DELPHI detector (section 5.1). If an off-energy electron coincides with the hadronic trigger, it can result in any of the following VSAT $\gamma$$\gamma$-event:

• If the off-energy electron is just in coincidence with a no-tag $\gamma$$\gamma$-event, it will be visible as a single tag.

• If the off-energy electron is in coincidence with a single tag $\gamma$$\gamma$-event, it can result in a double tag trigger.

• The coincidence of two off-energy electrons can also be triggered as a double tag $\gamma$$\gamma$-event.

The most troublesome background for this analysis is the second , as the confinement of the off-energy background is mixed up with the spread of true $\gamma$$\gamma$-events. The other two background triggers could in principle be completely removed by strong cuts, without too much loss of the signal. This is not possible for second type and the size of the background has to be estimated in order to make an efficiency calculation.